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The values h h h   and b b b   are called absolute cross
0 0
deformation of a rod.
The ratio of the absolute longitudinal deformation of a rod to its
original length is called relative longitudinal deformation:
l 
    .
поздов x (2.8)
l
0
The ratio of absolute cross deformation of a rod to its original
cross size is called relative cross deformation:

      h ,       b .
попер z попер y (2.9)
h b
0 0
For isotropic materials cross deformation are equal:   .
z y
The deformations  , and  are sometimes also called
x y z
linear deformations in the direction of ,x y and z axes.
The experiments proved that at low elongation between stresses
and deformations there is a direct proportional relationship:

  E . (2.10)
x x
This dependence is mathematically expressed Hooke's law in
tension and compression. The coefficient of proportionality E is
called elastic modulus or Young's modulus. Its dimensions –  .
Pа
Equating the right parts of (2.5) and (2.10) and taking into
account the relation (2.8), we obtain
N l
l   x 0 . (2.11)
EA
We see that the Young's modulus describes the ability of
materials to resist elastic deformation. The higher the value of the
modulus is, the less the rod stretches under other similar
l
l
conditions. If hypothetically imagine   that, from the
0
equation (2.11) E  N A  , i.e. Young's modulus is
x x
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